Currently, electrical appliances, such as a cellular telephone, a digital camera or the like, that incorporate rechargeable batteries are being widely used. When an AC adapter is connected to such an appliance, the appliance can operate on electric power supplied from an AC adapter, and at the same time the rechargeable battery can be charged. As such appliances have come into wider use, various AC adapters having different output voltages have come into use. Since limited types of connectors for the AC adapters are available, the AC adapters that output different voltages may have the same type of connector. Therefore, an improper AC adapter may be connected to an electrical appliance by mistake. If an AC adapter that outputs voltage higher than the endurance voltage of a semiconductor device in the electrical appliance is connected to the appliance, the semiconductor device may be damaged or broken.
In order to eliminate the above disadvantage, an excess voltage protection circuit shown in FIG. 1 has been developed (see Patent-related document listed below). In FIG. 1, an excess voltage protection circuit 113 is composed of resistors R111, R112 configured to divide the voltage output from an AC adapter 111, a resistor R113, a zener diode D111 that generates a reference voltage, a comparator 114 that compares the divided voltage obtained by dividing the voltage from the AC adapter 111 and the reference voltage, and a switching member M111 that is controlled to be turned on/off in accordance with the output voltage from the comparator 114. By the way, a resistor R114 serves as a bias resistor connected to the output terminal of the comparator 114, and a diode D112 indicates a parasitic diode of a PMOS transistor that composes the switching member M111.
When the AC adapter 111 that outputs an appropriate voltage is connected to the excess voltage protection circuit 113, electric potential is lower at an input IN1 than at an input IN2 of the comparator 114. In this case, the comparator 114 outputs a low level signal so as to cause the switch member M111 to be turned on, which allows the voltage output from the AC adapter 111 to be applied to an apparatus body 112. On the other hand, when the AC adapter 111 that outputs a higher voltage is accidentally connected to the excess voltage protection circuit 113, electric potential is higher at the input IN1 than the input IN2 of the comparator 114. In this case, the comparator 114 outputs a high level signal so as to cause the switch member M111 to be turned off, which prevents the higher voltage from being applied to the apparatus body 112.
FIG. 2 illustrates another related art excess voltage protection circuit (see Patent-related document 2 listed below). Referring to FIG. 2, the shown circuit includes an excess voltage protection circuit portion 121 and a CMOS IC circuit portion 122. The excess voltage protection circuit portion 121 and the CMOS IC circuit portion 122 share a semiconductor substrate. Terminals 131, 132, which are external terminals of the semiconductor apparatus, are connected to an external power source (not shown). Terminals 133, 134 are internal terminals of the semiconductor apparatus and serve as power source terminals for the CMOS Integrated Circuit (IC) 122. The excess voltage protection circuit portion 121 is composed of resistors R121, R122 that divide the power source voltage, a p-type Metal Oxide Semiconductor (PMOS) transistor M121 and a resistor R123, which compose an inverter, and a switching member M122.
When an appropriate voltage is applied across the terminals 131, 132, the voltage across the terminals of the resistor R122 is lower than a threshold voltage of the PMOS transistor M121. Therefore, the PMOS transistor M121 remains off and thus voltage across the resistor R123 is approximately 0 volts, which causes the switching member M122 composed of a PMOS transistor to remain on and the voltage input to the terminal 131 is then output to a terminal 133, thereby supplying electric power to the CMOS IC 122.
On the other hand, when a higher voltage is applied across the terminals 131, 132 and the voltage applied across the terminals of the resister R122 becomes larger than the threshold voltage, the PMOS transistor M121 is turned on. Then, the voltage across the resistor R123 becomes substantially equal to the voltage from the AC adapter 111, which causes the PMOS transistor M122 to be turned off, thereby stopping electric power from being supplied to the CMOS IC 122.
[Patent-related document 1] Japanese Patent Application Laid-Open Publication No. 2002-218645.
[Patent-related document 2] Japanese Patent Application Laid-Open Publication No. 2002-313949.
[Patent-related document 3] Japanese Patent Application Laid-Open Publication No. 2003-303890.